The present invention relates to apparatus for connecting sensors to a control or monitoring system, in particular, apparatus having fault-detecting capabilities.
As computer process control and monitoring plays a larger role in the industrial environment, the capability to connect large quantities of sensors efficiently and reliably to a central location has become increasingly important.
In the past, the process control or monitor for a machine or process had fewer input devices and sensors. In this situation, it was acceptable to wire individually each sensor to the control.
As control and monitoring has become more complicated due to automation needs, sensor inputs have multiplied and the number of wires required has grown very large.
Rather than run individual electrical circuits between each sensor and the control, methods have been used that distribute groups of I/O modules into remote electrical boxes such that each I/O group places sensor outputs onto shared electrical circuits or buses that multiplex the signals back to the control.
It is relatively easy and economical to protect the bus from physical damage because it contains a relatively small number of conductors or other data transmission medium, and it need only directly communicate with each I/O group which are typically located farther from physical hazards.
On the other hand, each sensor must still be located near the condition to be sensed. Unfortunately, it is often too expensive and/or difficult to protect fully the connection between the sensor and I/O group from physical hazards.
In practice, the connection between the sensors and the remote I/O group electrical box is often the portion of the control/monitoring system most likely to fail. Typically, this connection is in the form of flexible cables with a quick connector on one end, the other end being hard-wired. While tough and sturdy, these cables are often damaged in the harsh industrial environment.
As process control/monitoring becomes more complicated, it becomes increasingly critical to determine whether a condition received from a remote device is the actual state of the sensor or due to a fault in the sensor or its cable. Not only is it desirable to determine if a cable fault exists, it is also important to be able to identify quickly which cable is faulty.
To minimize costs and complexity, the invention contemplates that several sensors can be connected to a conveniently located concentrator, which does the necessary transformation to place the sensors information onto the bus. To increase flexibility, the concentrator uses, in one embodiment, standardized communications protocols.